Nonaqueous electrolyte secondary batteries for performing charge and discharge by using a nonaqueous electrolyte solution to transfer lithium ions between positive and negative electrodes are used as power sources for portable electronic devices, electric power storage, and others. In these nonaqueous electrolyte secondary batteries, graphite materials are widely used as the negative-electrode active material in the negative electrode.
Meanwhile, in recent years, size and weight reduction of mobile devices including cellular phones, notebook computers, and PDAs has rapidly progressed and the power consumption thereof has increased with increasing range of functions. Also for nonaqueous electrolyte secondary batteries used as power sources for the above mobile devices, there has been an increasing demand to reduce the weight and increase the capacity.
In order to increase the capacity of a nonaqueous electrolyte secondary battery, a technique is used in which a large amount of active material is packed into the battery to increase the packing density. In this case, however, the increase in packing density of the active material makes the electrolyte solution difficult to infiltrate into the entire region of the inside of the battery. Thus, the charge/discharge reaction will be nonuniform, which will easily cause local deterioration in the inside of the battery. Therefore, there is a need for an active material having a higher capacity than conventional materials. For the negative-electrode active material, a material having a higher capacity than graphite is being required.
For the above reason, studies have recently been conducted on the use of a material capable of alloying with lithium, such as silicon, germanium or tin, as the negative-electrode active material having a high capacity. With the use of such a material capable of alloying with lithium, the battery capacity can be increased but during alloying with lithium the volume of the negative-electrode active material is significantly increased by a charge reaction as compared with graphite materials and the like. At this time, the negative-electrode active material presses against the adjoining separator and positive-electrode active material layer, so that the electrolyte solution infiltrated in the inside of the electrode is squeezed out of the electrode assembly, resulting in decrease in the amount of electrolyte solution around the electrode. Thus, the charge/discharge reaction will be nonuniform, which will easily cause local deterioration in the inside of the battery.
In order to stabilize the battery characteristics, it is important to retain a state in which the electrolyte solution uniformly diffuses in the inside of the battery. Therefore, the electrolyte solution squeezed out of the electrode assembly needs to be infiltrated into the inside of the electrode assembly again. To this end, it is effective to reduce the viscosity of the electrolyte solution.
Generally, an electrolyte solution is composed of a solute and a solvent for dissolving the solute. Chain carbonates are used as common solvents and the content of chain carbonate in the electrolyte solution is relatively high. Therefore, if the viscosity of the chain carbonate is reduced, the viscosity of the electrolyte solution can also be reduced. For example, if diethyl carbonate commonly used is replaced with a chain carbonate having a small carbon number in the side chain, such as methyl ethyl carbonate or dimethyl carbonate, the viscosity can be reduced.
Alternatively, the use of a carboxylic acid ester or a ketone exhibiting a lower viscosity than chain carbonates can further reduce the viscosity of the electrolyte solution.
However, low-viscosity chain carbonates, carboxylic acid esters, and ketone shave relatively narrow potential windows owing to their small molecular weights and high reactivity. Thus, the nonaqueous electrolyte solution will be electrochemically unstable, will easily cause side reactions with the active material, and tends to degrade the battery characteristics. With the use of materials capable of alloying with lithium, such as silicon, as the negative-electrode active material, these materials easily react particularly with the electrolyte solution, which presents a problem in that the battery characteristics will be more significantly degraded.
In addition, when the battery is stored in a charged state in a high-temperature environment, the reaction of the above material with the electrolyte solution will be particularly significant and the attendant gassing and like present some problems, such as increase in thickness of the electrode.
Patent Literature 1 discloses that the addition of a small amount of fluorobenzene, cyclohexylbenzene or cyclohexylfluorobenzene to the electrolyte solution enables suppression of the reaction of the negative-electrode active material, such as silicon, with the nonaqueous electrolyte solution. However, there is a demand to further suppress the reaction with the electrolyte solution and further enhance the charge-discharge cycle characteristic.
The present invention employs a nonaqueous electrolyte solution containing benzotrifluoride and a diisocyanate compound as will be described later.
Patent Literature 2 discloses a nonaqueous electrolyte secondary battery in which an electrolyte solution containing a diisocyanate compound is used. However, the literature does not disclose any effect that would be caused if the diisocyanate compound were used together with benzotrifluoride.